I have the same question. I have a polar watch that does VO2 but obviously, that is not going to match professional testing. It may however, provide a rough guide as to improvement. I am interested in what others do.

My Garmin watch (Vivoactive 4) has an algorithm that calculates VO2 Max based on a host of variables that it monitors and your age and weight, which you input.

The “gold standard” for measuring VO2 Max uses a treadmill and a number of monitoring devices to which they hook you up in a lab setting. This is expensive and strenuous…I’ve never done this.

There may be differences between the lab derived number and the Garmin algorithm; but the most important factor is the observed trend, so the Garmin derived number is more than adequate to monitor trends.

There are several ways to determine your VO2 Max; without a Garmin watch or a lab visit.

The least strenuous method to estimate your VO2 max is to briskly WALK one mile, recording your time to do so (with stopwatch) and immediately determining your heart rate AFTER the brisk one mile walk; then inputting that data, along with your age and weight, into the calculator at the website below.
https://www.trainermetrics.com/fitness-assessment-calculations/vo2max-one-mile-walk/